This invention relates to a method and apparatus for forming elongate tubular members into predetermined shapes. More extreme shapes are made possible by pressurizing the interior of the tubular member with a gas prior to the shaping step to provide greater resistance to crimping and kinking as the shaping takes place. The particular disclosure of the invention contained in this application for purposes of illustration relates to a metal extrusion formed of aluminum such as would be used for the corner post of an automobile body. However, tubular structures fabricated by other methods can also be formed according to the invention disclosed.
The forming process takes place by first placing the extrusion under sufficient tension to cause the material of the extrusion under sufficient tension to cause the material of the extrusion to enter its "yield state" where it is particularly susceptible to shape alteration without crimping or other undesirable effects. Such structures in the past have nevertheless been susceptible to sidewall crimping or kinking past a relatively slight degree of bending. This is particularly true of extrusions having relatively thin walls. As weight has become a more important consideration in the manufacture of many products, such as automobiles using such structures, the walls of extruded structural members have become thinner. The increased tendency of the extrusion walls to crimp or kink beyond the point of a relatively shallow bend has limited the use of thin wall extrusions.
The problem posed, therefore, is to make the walls of an extrusion resistant to crimping or kinking without increasing the thickness of the walls.
The solution achieved must not interfere with the formation of the extrusion into its predetermined shape, prolong the formation process or otherwise impair the quality of the extrusion. The solution must also take place within the environment of the extrusion forming process. This process includes the steps of holding the extrusion by its opposite ends in jaws carried by opposing tension cylinders which are free to pivot so as to maintain a tangent relation to the part being formed. A die is urged against the extrusion while tensioned in its yield state, causing the extrusion to assume a new shape. The shape may define a symmetrical or an asymmetrical shape and may have compound curves. After the shape is formed, tension on the extrusion is reduced so that the extrusion is released from its yield state, and then the extrusion is removed from the jaws of the extrusion former.